Please suggest me a programming language for a new, vertical business application [on hold]

We need to develop a large enough business application for our company. I estimate the amount of work it will require is about 1 man year at a minimum.

The application consists of a series of interconnected subsystem communicating through a VPN. There are some already running portions (Ubuntu web server, web backend, orders and other business data are stored on a MySQL server), others are almost ready (database replication), others are ready (some end user Windows applications).

We need to implement a main software to replace our current, legacy one. It's very specialistic, it got hundreds of fancy business rules so it's not possible to really re-use something already made. It has some "classic" features though, including customers records, orders history and similar. It will feature reporting, charting, PDF generation and so on.

The CEO wants this new vertical software to run on a Windows server located in a data center, which makes everything a bit harder. We'll have to manage all sorts of typical network discontinuities / failures, we have other remote servers to deal with anyway.

Initially, just 5 users would access the application, in the future the number could go up to 30. It's never going to scale a lot higher than that.

Now, the big question is: what modern programming language would be good for this?

We have a very strong knowledge of Delphi (up to XE 5), some C# knowledge. But we are not afraid to acquire and learn whatever would be needed for this project. Delphi would afford us a "quick" development time (below one year) but it's also a legacy platform with all sorts of issues and challenges, expecially when talking about remote communications and multi-threading.

I am looking for informed suggestions about which technology and language / platform we could use to develop this software in a as smooth as possible way.

Thanks in advance.